Transcript IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L15.1: Optimization of protection in radiography: technical.

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L15.1: Optimization of protection in radiography:
technical aspects
IAEA
International Atomic Energy Agency
Topics
 Intensifying screen structure and characteristics
 Screen-film combination
 Radiographic film structure and characteristics
 Antiscatter grid
 Film processor
 Darkroom and View Box
 Image parameters
IAEA
15.1: Optimization of protection in radiography: technical aspects
2
Overview
• To become familiar with basic knowledge of
the components that form the radiographic
chain.
IAEA
15.1: Optimization of protection in radiography: technical aspects
3
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 1: Intensifying screen structure and
characteristics
IAEA
International Atomic Energy Agency
Primary beam attenuation and latent image
Film, fluorescent screen
or image intensifier
Scattered
radiation
« Latent »
radiological
image
Bone
X
Soft
tissue
Air
Primary
collimation
Antiscatter Grid
IAEA
Beam intensity
at detector level
15.1: Optimization of protection in radiography: technical aspects
5
Intensifying screen
•
•
•
•
Layer of material placed immediately adjacent to film in
conventional radiography to:
Convert the incident X Rays into radiation more suitable
for the light-sensitive emulsion of the radiographic film (X
Ray  light photons)
Reduce the patient dose needed to achieve a given level
of film quality
Reduce the exposure time as well as the power required
from the X Ray generator (cost savings)
Increase photoelectric effect  better use of the beam
energy (image formation)
IAEA
15.1: Optimization of protection in radiography: technical aspects
6
Intensifying screen structure (I)
• Supporting Base (mainly polyester material)
• chemically neutral, resistant to X Ray exposure, flexible
• Reflecting layer (Titanium dioxide - TiO2)
• a crystalline compound reflecting photons toward
sensitive emulsion
• Fluorescent layer (polymer)
• crystals dispersed in a suspension of plastic material
• Protective overcoat
• colourless thin overcoat to help avoid abrasions of
fluorescent layer due to the use of screen
IAEA
15.1: Optimization of protection in radiography: technical aspects
7
Intensifying screen structure (II)
(Incident X Ray beam)
Supporting Base (240 m)
Screen
Reflecting layer (25 m)
Fluorescent layer (100 to 400 m)
Protective overcoat (20 m)
(Light-sensitive film)
IAEA
15.1: Optimization of protection in radiography: technical aspects
8
Intensifying screen structure (III)
• The fluorescent layer
• should:
• be able to absorb the maximum quantity of X Rays
• convert the X Ray energy into light energy
• match its fluorescence with the film sensitivity (color of
emitted light)
• Type of material:
• Calcium tungstate CaWO4 till 1972
• Rare earth since 1970 LaOBr:Tb and Gd2O2S:Tb
 more sensitive and effective than CaWO4
IAEA
15.1: Optimization of protection in radiography: technical aspects
9
Intensifying screen characteristics (I)
• IF (Intensification Factor): ratio of exposures giving
the same film optical density, with and without
screen
• 50 < IF < 150 (depending on screen material and X Ray beam
energy)
• QDE (Quantum Detection Efficiency): fraction of
photons absorbed by the screen
• 40% for CaWO4 < QDE < 75% for rare earth (depending on
crystal material, thickness of fluorescent layer and X Ray
spectrum)
IAEA
15.1: Optimization of protection in radiography: technical aspects
10
Intensifying screen characteristics (II)
•  Conversion efficiency—
ratio of light energy emitted to X Ray energy
absorbed (%)
• 3% for CaWO4 <  < 20% for rare earth
• C (Detection Coefficient): ratio of energy captured
and used by the film to energy emitted by the
crystal (%)
• C is maximum for screens emitting in UV wave length  90%
IAEA
15.1: Optimization of protection in radiography: technical aspects
11
Relative Sensitivity of Film
Intensifying screen characteristics (III)
Sensitivity of a Conventional Film
BaSO4:Eu,Sr
YTaO4:Nb
BaSO4:Pb
CaWO4
250
IAEA
300
UV
350
400
Blue
450
500
550
Green
15.1: Optimization of protection in radiography: technical aspects
600
12
Intensifying screen characteristics
(IV)
• Intensifying factor: ratio of exposures giving the
same film optical density, with and without screen
175
150
Gd2O2S
125
100 LaOBr
75
50
CaWO4
25
0
50 60 70 80 90 100 110 120
IAEA
15.1: Optimization of protection in radiography: technical aspects
kV
13
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 2: Screen film combination
IAEA
International Atomic Energy Agency
Screen film combination
• Sensitivity (screen film): The quotient K0/Ka, where K0 = 1
•
•
•
•
mGy and Ka is the air kerma free-in-air for the net density
D = 1.0, measured in the film plane
Screen film system: A particular intensifying screen used
with a particular type of film
Sensitivity class: Defined range of sensitivity values of a
screen film system
Single emulsion film: One coated film used with one
intensifying screen
Double emulsion film: A double coated film used with a
couple of intensifying screens
IAEA
15.1: Optimization of protection in radiography: technical aspects
15
Screen film combination performance
• Spatial Resolution: capability of a screen film combination
to record and display a test pattern specified in cycles/mm.
Modulation Transfer Function (MTF): description of how
sinusoidal fluctuations in X Ray transmission through the
screen film combination are reproduced in the image
• Noise spectrum: Noise as a function of frequency
• Quantum Detection Efficiency (QDE): Measure of
combined effect of signal and noise performance as a
function of frequency
IAEA
15.1: Optimization of protection in radiography: technical aspects
16
Screen film combination performance
• Assure that screen emission spectrum matches
sensitivity of film being used
• Screen film contact
• loss of spatial resolution
• blurred image
• Cleanliness
• Inter cassette sensitivity
IAEA
15.1: Optimization of protection in radiography: technical aspects
17
Effect of screen on resolution
• Screen resolution is dependent on the crystal size and
thickness of screen
• Direct exposure radiography has better resolution than
screen-film (but requires around 40 times the radiation
exposure)
• Direct exposure film ~ 30 c/mm; 200 speed screen-film
system ~ 10 c/mm; 400 screen-film system ~ 6 c/mm;
mammography system ~ 15 c/mm
IAEA
15.1: Optimization of protection in radiography: technical aspects
18
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 3: Radiographic film structure, image
formation and processing characteristics
IAEA
International Atomic Energy Agency
Radiographic film
(structure and characteristics)
• Protective layer (outer surface)
• Sensitive layer (~20µ)
• Base material (transparency and mechanical
resistance) (~170µ)
• Binding (base-sensitive layer) or anti cross-over
layer
• Filtering layer
• Sensitivity class
IAEA
15.1: Optimization of protection in radiography: technical aspects
20
Radiographic film structure
Supercoat
Emulsion (~5-20 µm thick)
Adhesive layer
Base (~200 µm thick)
Anti-curl,
anti-halation layer
Single Emulsion Film
IAEA
15.1: Optimization of protection in radiography: technical aspects
21
Film construction
• Supercoat - prevents scratching
• Base
• provides relatively thick, semi-rigid structure to film, but
still allowing flexibility
• almost (but not completely) transparent
• Emulsion
• image layer, composed of gelatine and silver halide (Br,
I) crystals in ionic form
• speed,contrast, resolution varied in emulsion
IAEA
15.1: Optimization of protection in radiography: technical aspects
22
Radiographic film structure
Supercoat
Emulsion
Adhesive layer
Base
Adhesive layer
Emulsion
Supercoat
Double Emulsion Film
IAEA
15.1: Optimization of protection in radiography: technical aspects
23
Silver halide reaction
• Latent image (invisible) formed by
interaction of a light photon from screen,
with a halide ion within the crystals, which:
• releases an electron,
• which in turn reacts with silver ion,
• forming atomic silver within the crystal
IAEA
15.1: Optimization of protection in radiography: technical aspects
24
Processing
• Development
• Converts latent image to metallic silver
• Fixing
• Dissolves unexposed silver halide crystals,
leaving only metallic silver, creating a
permanent image
IAEA
15.1: Optimization of protection in radiography: technical aspects
25
Steps in image formation
IAEA
15.1: Optimization of protection in radiography: technical aspects
26
Spectral response and spectral
matching
• The variation in film sensitivity to the various
colours of light
• Film is usually blue or blue-green sensitive
(orthochromatic)
• Screens emit blue (e.g., calcium tungstate)
or green (rare earth screens) light
• Safelights must not affect film
IAEA
15.1: Optimization of protection in radiography: technical aspects
27
Spectral response of film
IAEA
15.1: Optimization of protection in radiography: technical aspects
28
Crossover
• In double emulsion film, light emitted by one
screen can cross over through the adjacent
emulsion, and the base and expose the
second emulsion
• This will reduce the resolution of the image
• Is prevented with a light-absorbing dye layer
IAEA
15.1: Optimization of protection in radiography: technical aspects
29
Crossover
IAEA
15.1: Optimization of protection in radiography: technical aspects
30
Optical density
Transmitted
light intensity
Incident light
intensity
It
I0
Optical Density = log10 I0 / It
Film
IAEA
e.g. 10% transmission = 1.00
1% transmission = 2.00
15.1: Optimization of protection in radiography: technical aspects
31
Characteristic curve of a radiographic
film
Optical
Density (OD)
Saturation
OD2
Visually evaluable
range of densities

OD1
Base
+ fog
IAEA
 = (OD2 - OD1) / (log E2 - log E1)
The  of a film: the
gradient of the «straight
line» portion of the
Normal range characteristic curve
of exposures
E1
E2
Log Exposure (mR)
15.1: Optimization of protection in radiography: technical aspects
32
Average gradient
• The straight line portion of the characteristic curve
is difficult to determine (and there may not be one),
so the average gradient is measured between
optical densities of 0.25 and 2.00
IAEA
15.1: Optimization of protection in radiography: technical aspects
33
Film sensitometry parameters
• Base + fog: The optical density of a film due to its base
density plus any action of the developer on the unexposed
silver halide crystals usually 0.15 -0.30.
• Sensitivity (speed): The reciprocal of the exposure value
needed to achieve a film net optical density of 1.00
• Gamma (contrast): The average gradient of
the characteristic curve
• Latitude: The range of exposures that can be recorded
and visualized on the film.
IAEA
15.1: Optimization of protection in radiography: technical aspects
34
Comparison of characteristic curves
OD
OD
Film
A
Film
B
Film
A
1+B+Fog
Log Exposure (mR)
Film A is faster
than Film B
Film A and B
have the same
contrast
IAEA
Film
B
Log Exposure (mR)
Film A and B
have the same
sensitivity but
different
contrast
15.1: Optimization of protection in radiography: technical aspects
35
Sensitometric strip
IAEA
15.1: Optimization of protection in radiography: technical aspects
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Sensitometry: A method of exposing a film by
means of a light sensitometer and assessing
its response to exposure and development
36
Sensitometric strip
IAEA
15.1: Optimization of protection in radiography: technical aspects
37
Latitude
Film B has higher latitude
(range of useful exposures)
than film A, but has lower
contrast (slope of the
curve)
IAEA
15.1: Optimization of protection in radiography: technical aspects
38
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 4: Anti-scatter grid and grid performance
parameters
IAEA
International Atomic Energy Agency
Anti-scatter grid (I)
• Radiation emerging from the patient
• primary beam: contributing to the image formation
• scattered radiation: reduces contrast
• the grid (between patient and film) eliminates most of
•
•
•
•
scattered radiation
stationary grid
moving grid (better performance)
focused grid
Potter-Bucky system
IAEA
15.1: Optimization of protection in radiography: technical aspects
40
Anti-scatter grid (II)
Source of X Rays
Patient
Scattered X Rays
Lead strip
Useful X Rays
IAEA
Film and cassette
15.1: Optimization of protection in radiography: technical aspects
41
Grid performance parameters (I)
• Grid ratio
• Ratio of the height of the strips to the width of the gaps at the
central line
• Contrast improvement ratio
• Ratio of the transmission of primary radiation to the
transmission of total radiation
• Grid exposure factor
• Ratio of the total radiation without the anti-scatter grid in a
specified radiation beam to that with the anti-scatter grid placed
in the beam
IAEA
15.1: Optimization of protection in radiography: technical aspects
42
Grid performance parameters (II)
• Strip number
• The number of attenuating lead strips per cm
• Grid focusing distance
• Distance between the front of a focused grid and the line formed
by the converging attenuating lead strips of the grid
IAEA
15.1: Optimization of protection in radiography: technical aspects
43
Example of anti-scatter grids (grid
ratio)
Grid: C
Grid: A
Grid: B
D
h


1
h
Grid ratio: r =
= tg

D

5 < r < 16
• Grid A and B have the same strip number
• Grid B and C have the same interspace between the lamella
IAEA
15.1: Optimization of protection in radiography: technical aspects
44
Grid selectivity(I)
Grid: C
Grid: A

IAEA
Grid: B


15.1: Optimization of protection in radiography: technical aspects
45
Grid selectivity (II)
% of scattered beam transmitted
100
90
80
70
60
55
50
45
40
35
30
25
20
15
10
5
0
• A grid with r = 12 transmits 5%
of scattered radiation
• A grid with r = 16 transmits 3.8%
30% difference in patient dose
5%
3.8%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
IAEA
15.1: Optimization of protection in radiography: technical aspects
r
46
Grid focusing error
(virtual increasing of grid shadow)
X Ray source
(too far)
X Ray source
(too close)
Grid
Film and cassette
grid shadow deformation
(applicable to both cases)
IAEA
15.1: Optimization of protection in radiography: technical aspects
47
Grid focusing error
(leading to 25% of beam loss)
GRID
CHARACTERISTICS
Shortest
distance
Longest
distance
Focalization
(cm)
80
Ratio
r
7
(cm)
(cm)
68
96
80
10
72
91
100
10
87
116
100
14
91
110
150
13
130
180
IAEA
15.1: Optimization of protection in radiography: technical aspects
48
Grid out of center
(virtual deformation of grid shadow)
X Ray source
Lateral shift
Film and
cassette
Grid
Grid shadow
IAEA
15.1: Optimization of protection in radiography: technical aspects
49
Grid focusing error due to lateral shift
(leading to 25% loss of X Ray beam)
GRID
CHARACTERISTICS
Focalization Ratio
(cm)
r
80
7
IAEA
MAXIMUM
LATERAL SHIFT
(cm)
2.8
80
10
2
100
10
2.5
100
14
1.8
150
13
2.9
15.1: Optimization of protection in radiography: technical aspects
50
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 5: Film processor
IAEA
International Atomic Energy Agency
The automatic film processor
IAEA
15.1: Optimization of protection in radiography: technical aspects
52
Automatic processors
•
•
•
•
Constant temperature
Constant processing time
Automatic replenishment of chemicals
Drying of films
BUT
• Can introduce artifacts
IAEA
15.1: Optimization of protection in radiography: technical aspects
53
Film processor QC
• Most important QC features:
•
•
•
•
•
•
•
proper film storage
darkroom cleanliness
cassette and screen care
processor chemical care
Sensitometry and processor quality control
artifacts
processor cleanliness
IAEA
15.1: Optimization of protection in radiography: technical aspects
54
Sensitometry (I)
•
•
•
•
Sensitometer and densitometer required
Essential - to keep film processing under control
To be performed daily
Values to be controlled:
• base + fog
• mid-density
• density difference
IAEA
15.1: Optimization of protection in radiography: technical aspects
55
Sensitometry (II)
• Use a sensitometer to expose a film to light
through the special step wedge
• Ensure that the emulsion side of the film (if
single emulsion) is toward the light source
• Select the correct light colour (green, blue)
on the sensitometer (if selectable), and
expose until the signal shows the exposure
is complete
• Process the film immediately
IAEA
15.1: Optimization of protection in radiography: technical aspects
56
Processor quality control
• The base-plus-fog level, mid density, and
density difference should be plotted on
control charts (Reference Gray, et al., ACR
Mammography Quality Control Manual)
IAEA
15.1: Optimization of protection in radiography: technical aspects
57
Manual Processing
• There are many places where X Ray films
are processed manually, in open tanks
• Manual processing can be very effective,
BUT there can be many quality problems
• It is essential that the developer temperature
be controlled and that the development time
be selected based on the temperature
IAEA
15.1: Optimization of protection in radiography: technical aspects
58
Dark room conditions in some
hospitals
IAEA
15.1: Optimization of protection in radiography: technical aspects
59
Film Processing
• Film processing includes:
• developer
• water wash
• fixer
• water wash
• Washing is very important to avoid chemical
contamination, and to assure archival properties of
the image
IAEA
15.1: Optimization of protection in radiography: technical aspects
60
Basic Film Processing Requirements
• Temperature - constant and optimum
(recommended by the film manufacturer)
• Time – measured, based on developer
temperature
• Developer activity (chemical condition) Properly replenished developer and fixer
IAEA
15.1: Optimization of protection in radiography: technical aspects
61
Temperature (I)
• The temperature of the developer should be
as recommended by the manufacturer
• Use a thermometer to check the
temperature before processing
IAEA
15.1: Optimization of protection in radiography: technical aspects
62
Maintaining temperature
• Ideally both developer and fixer containers should
be surrounded by a water bath (as a thermal
jacket)
• This water bath should maintained at the
temperature specified by the film manufacturer
IAEA
15.1: Optimization of protection in radiography: technical aspects
63
Manual processing tanks
Water bath
surrounding
tanks (not
filled here)
IAEA
15.1: Optimization of protection in radiography: technical aspects
64
Chemical activity
• The correct chemicals for manual
processing must be used
• Chemicals should be replenished daily
based on the film (chemical)
manufacturer’s instructions
IAEA
15.1: Optimization of protection in radiography: technical aspects
65
Chemical activity
• Films must be agitated every 20 seconds
during development and fixing.
• Once film is developed the film is washed
in clean water before being put in the
fixer.
• Never put films from the fixer back in the
developer
• Avoid splashing fixer into the developer
container.
IAEA
15.1: Optimization of protection in radiography: technical aspects
66
Chemical activity (IV)
• As films are developed, the developer and
fixer chemicals become depleted
• Compensate for this by proper replenishment
of the chemicals
• Also, air will oxidise the developer (making it
turn brown)
• Both will cause underdevelopment and poor
quality X Ray films
IAEA
15.1: Optimization of protection in radiography: technical aspects
67
Measuring Chemical Activity (I)
• Use of a sensitometer is preferred with
the use of a densitometer
• However, much can be done with a
standard ‘phantom’ and viewing box
• Standard ‘phantom’ could be a
• Step wedge
• Uniform block of acrylic at least 20 cm
thick
IAEA
15.1: Optimization of protection in radiography: technical aspects
68
Measuring Chemical Activity (II)
• Procedure
• Start with properly mixed, fresh chemicals at the
temperature specified by the manufacturer
• Expose object at a set kVp, mAs and focus to film
distance
• record these factors for future use
• always use the same factors for test film
• Process film (using correct temperature and
processing time) and use as reference
• compare processor check film with standard film to
check chemical activity
IAEA
15.1: Optimization of protection in radiography: technical aspects
69
Measuring Chemical Activity (III)
• Signs that developer activity is low
• Loss of film contrast
• Loss of overall film density
• Replace developer and fixer (at the
same time) if developer activity is low
IAEA
15.1: Optimization of protection in radiography: technical aspects
70
Measuring Chemical Activity (IV)
• Signs that fixer activity is low
• films take longer to ‘clear’
• Replace fixer if activity is low
IAEA
15.1: Optimization of protection in radiography: technical aspects
71
Washing
• Films must be rinsed briefly but
thoroughly between developer and fixer,
• And washed for 30 minutes following
fixing, to clear all traces of fixer (which
can degrade the X Ray over time)
• There should be a continuous flow of
water through the wash tank at the
same temperature as the developer and
fixed
IAEA
15.1: Optimization of protection in radiography: technical aspects
72
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 6: Darkroom and view box
IAEA
International Atomic Energy Agency
Darkroom characteristics
• Safelight
• number (as low as possible),
correct distance from the
table
• type and colours of filters
• power ( 15 W or less)
• No light leaks from outside
• Room temperature < 20°22°
• Film storage conditions
IAEA
15.1: Optimization of protection in radiography: technical aspects
74
Viewbox characteristics
Since the viewing conditions are essential for a good
interpretation of the diagnostic images, the viewing
conditions must be optimal
• Cleanliness of external and internal surface
• Brightness (luminance)
• homogeneity of different viewing boxes: 1300 2000 cd/m2
• homogeneity within the same viewing box
• Colour
• colour mismatch must be avoided
• Environment (illuminance)
• ambient light level: 50 lux maximum
IAEA
15.1: Optimization of protection in radiography: technical aspects
75
Viewing box brightness
5700
5810
5610
6200
5920
EXAMPLE OF
MEASUREMENTS
IAEA
6110
6130
5860
6090
5920
CORRECT
CONFIGURATION
(cd/m2)
15.1: Optimization of protection in radiography: technical aspects
76
Viewing box color and brightness
5700
5810
3510
6200
5920
3870
BLUE
COLOR
4160
5860
2150
WHITE
COLOR
3110
WRONG
CONFIGURATIONS
(cd/m2)
IAEA
15.1: Optimization of protection in radiography: technical aspects
77
Measurement of Luminance
Units: cd.m-2
IAEA
15.1: Optimization of protection in radiography: technical aspects
78
Measurement of Illuminance
Units: lux
IAEA
15.1: Optimization of protection in radiography: technical aspects
79
Example of poor viewing box
IAEA
15.1: Optimization of protection in radiography: technical aspects
80
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 15.1: Optimization of protection in
radiography
Topic 7: More image quality characteristics
IAEA
International Atomic Energy Agency
Image quality characteristics
•
•
•
•
•
•
•
Density
Contrast
Resolution
Unsharpness
Noise
Distortion
MTF
IAEA
15.1: Optimization of protection in radiography: technical aspects
82
Factors characterising film quality
Film
Geometry
Subject
Density,
contrast,
speed,
latitude
Distortion,
magnification,
blur (unsharpness)
Contrast
(thickness,
density,
atomic number)
Processing
IAEA
Motion
15.1: Optimization of protection in radiography: technical aspects
83
Contrast
• The difference between the optical density in two
parts of a radiographic image
• Made up from two sources:
• 1. Subject contrast: the different amounts of radiation
exiting different parts of the body
• Affected by tissue density, atomic number and
density, X Ray energy (kVp), scatter
• 2. Detector contrast: made up of the properties of the
detector (e.g., screen-film system and processing)
IAEA
15.1: Optimization of protection in radiography: technical aspects
84
Subject Contrast (1)
IAEA
15.1: Optimization of protection in radiography: technical aspects
85
Subject Contrast (2)
• Apart from the patient, the important
factors are kVp and scatter
• High kVp means higher penetration
and less variation in absorption in
body tissues, and thus lower contrast
• Low kVp gives more differential
absorption and thus high contrast (we
use low kVp for mammography)
IAEA
15.1: Optimization of protection in radiography: technical aspects
86
Subject contrast (3)
• Scattered radiation can significantly
reduce contrast, and is reduced with a
grid
IAEA
15.1: Optimization of protection in radiography: technical aspects
87
Subject contrast (4)
• Grid performance can be described by
the radiographic contrast improvement
ratio k
k = (Image contrast with grid)/(contrast
without grid)
• k is normally between 1.5 and 2.5
• Subject contrast can be improved by
using iodine- or barium-containing
contrast agents in the patient
IAEA
15.1: Optimization of protection in radiography: technical aspects
88
rison of characteristic
Detector contrast
(OD)
Film
B
Log
A is faster
n Film B
A and B
the same
IAEA
ontrast
Film
A
Film
B
Log Exposure
Film A and B
have different
contrast
15.1: Optimization of protection in radiography: technical aspects
89
Resolution and unsharpness
• Spatial resolution (or image blur) is
the ability to distinguish closely
spaced objects
• Resolution is measured in a number
of ways, but most commonly as
cycles per millimeter (c/mm)
• The higher the cycles per millimeter,
the better the resolution
IAEA
15.1: Optimization of protection in radiography: technical aspects
90
Spatial resolution
• Resolution is affected by a number of
factors:
• focal spot size
• type intensifying screen
• motion
• image noise
IAEA
15.1: Optimization of protection in radiography: technical aspects
91
Effect of focal spot on resolution
Resolution and the Focal Spot
Focal spot
Object
Foca l spot
blur
More blur
Appearance of image
F lu or os co p y R ad iatio n S afe ty - Lee Co llins , J u ly 2 0 01
IAEA
15.1: Optimization of protection in radiography: technical aspects
Less blur
62
92
Noise (1)
• The fluctuation of optical density in the
image over very small distances
• Some noise is inherent in the imaging
system, some is controllable
IAEA
15.1: Optimization of protection in radiography: technical aspects
93
Noise (2)
• Noise is mostly caused by:
1. The number of X Ray photons used
in the image (quantum mottle) most important component
2. The limited absorption efficiency of
X Rays by the screen (structure
mottle)
3. The crystal size and distribution in
film (film graininess)
IAEA
15.1: Optimization of protection in radiography: technical aspects
94
Magnification
• The larger the gap between the object
and the image receptor, the more the
image will be magnified
Magnification =
image size/object size
= SID/SOD
Object
SOD
Image
SID
IAEA
15.1: Optimization of protection in radiography: technical aspects
95
Summary
• The main components of the radiography
chain and their respective role are
explained:
• conventional film and screen-film combination
characteristics
• required conditions for film processing
(darkroom) and image viewing (view box)
IAEA
15.1: Optimization of protection in radiography: technical aspects
96
References
• The Essential Physics of Medical Imaging. JT Bushberg, JA
Seibert, EM Leidholdt, JM Boone. Lippincott Williams &
Wilkins, Philadelphia, 2011
• The physics of diagnostic imaging, Dowsett et al, Hodder
Arnold, 2006
• Quality Control in Diagnostic Radiology, Gray JE. et al.
http://diquad.com/QC%20Book.html
• Mammography quality control: Radiologic technologists
manual. American College of Radiology, Reston, VA. 1999
IAEA
15.1: Optimization of protection in radiography: technical aspects
97